
: 
| 
oe ee eee eee 
















_ which appear to go on within the chemical spectrum. 
_ tends up to 2°20. The inner portion of this spectrum 
much more intense over this section than on any other 
_ the complete production of metallic silver in a state of 
brown edge, but does not act in the slightest degree on 
| the inside oval space. The spectrum obtained without 
f any interposed medium is, upon collodion, of less extent 
than that now described; it is, usually, a long flame- 
ON THE CHEMICAL ACTION OF THE SOLAR RADIATIONS. 271 
with full intensity to 1°15. This forms a well-defined dark olive-green oval 
spot; it is somewhat smaller at the upper end; the chemical action is then 
continued faintly to 1-35, and still more faintly to 1°60. At the lower end the 
impressed image descends to ‘50, and a faint border of dusky brown sur- 
rounds the spectrum, which is impressed over a'space equal to 1°5. The prin- 
cipal action is limited to the blue and the rays above it. , 
(61) 33. INTENSE GREEN (20), fig. 29.—The action hereon _ Fig. 29. 
the most sensitive collodion plates is exceedingly slow, and after © 
an exposure of five minutes in the brightest sunshine, the only ——_——* 
indication of any chemical action is the appearance of a faint 
spot near the line H. This when acted on by the pyrogallic 6 
acid becomes very dark, and another spot a little beyond the 
violet rays makes its appearance. 
(62). In the camera obscura, which has been devised for 
working with the very sensitive and beautiful collodion process 
in the open air, yellow glasses have been introduced for the a’ 
purpose, as it was thought, of cutting off the chemical rays, at 
the same time as light enough was admitted to enable the operator to see 
his work. The results obtained (see Yellow Media) clearly prove that rays, 
chemically active for collodion, pass the yellow media very freely; some 
green glasses, as the above, offer much more obstruction, but red glasses ap- 
pear to be still more effective. c 
(63) 44. DEEP IRON-GREEN (27).—The action of this spectrum is very 
slow, and confined to the limits between the mean green ray and the extreme 
violet. In agreat many experiments the spectrum impressed has been always 
limited to the space °50; that is, it has commenced in the middle of the 
luminous spectrum and terminated with the violet rays. Very weak hypo- 
sulphite of soda washes off the darkened portion so readily, that I am led to 
infer that it is an exceedingly superficial dust upon the surface only. 
(64) 116. ANOTHER DEEP IRON-GREEN (25).—By long-continued action 
there is scarcely a trace of any chemical change. Here we have an example 
of a spectrum in which the blue class of rays, ordinarily called the chemical 
rays, are very brilliant, yet they are chemically inactive upon this most sen- 
sitive photographic preparation. 
Fig. 30. 
Glasses (D.) Blue Media. 
(65) 105. DEEP COBALT-BLUE (38), fig. 30.— The 
rapidity of action with this medium prevents the marking 
of many of the more remarkable gradations of change 
Chemical action commences between °70 and 1°70, and ex- 
appears the lightest, but in reality ‘the action has been 
part, and the semi-transparency of this portion is due to 
fine division. Ammonia will dissolve off the outer dark 

shaped band of 1°60 or 1°70 in length, and of uniform : 
intensity throughout. The operation of the cobalt-blue we 
